The present invention relates to an optical module having an element mounted thereon for shifting an optical phase using the electro-optic effect.
Optical communication systems have been introduced to implement large-capacity broadband communication systems. As demands for expanding communication capacity increase, higher bit rate is desired in such optical communication systems.
Meanwhile, in optical communication systems, there has been employed an optical module as an optical modulator, a beam polarizer and an electric shutter, on which an element capable of shifting an optical phase by the electro-optic effect is mounted. Here, the electro-optic effect is a known effect by which a refractive index of a ferroelectric crystal or the like is varied when electric field is applied thereupon.
The element for shifting an optical phase by the electro-optic effect (hereinafter simply referred to as electro-optic effect element) mounted on such an optical module includes an optical waveguide. The optical waveguide is formed by patterning a metal film of Ti, etc. on a wafer being cut from an electro-optic crystal formed of LiNbO3, LiTaO2, etc. and then by thermal-diffusing or by proton-exchanging in benzoic acid for use in IC manufacturing process. Also, necessary electrodes are produced near the optical waveguide.
The optical module is configured with an optical signal supplied from outside the electro-optic effect element to the optical waveguide and a high frequency control signal of microwave band being supplied to the electrodes produced near the optical waveguide.
FIG. 1 shows a top plan view of a configuration example of the optical module for use in an optical modulator in an uncovered state. An electro-optic effect element 2 is accommodated in a shielding case 1. FIG. 2 shows a schematic configuration diagram of electro-optic effect element 2.
To function as an optical modulator, an optical waveguide 10 formed on electro-optic effect element 2 schematically illustrated in FIGS. 2A to 2C is, as one example, split into a pair of branching waveguides disposed in parallel to form a Mach-Zehnder waveguide. FIG. 2B is a cross-sectional view along line xe2x80x98axe2x80x99 in the plan view shown in FIG. 2A, while FIG. 2C is a cross-sectional view along line xe2x80x98bxe2x80x99.
In an exemplary configuration in which a single electrode is employed in the intensity modulation scheme using a Z-cut wafer which is produced by cutting a wafer of electro-optic effect element 2 in Z-axis direction of LiNbO3 crystal, a signal electrode 20xe2x80x2 is disposed right on one branching waveguide out of the aforementioned parallel waveguides, while a ground electrode 22 is disposed right on the other branching waveguide. Further, a buffer layer formed of SiO2 is provided between the wafer and signal electrode 20 and between the wafer and ground electrode 22, so that an optical signal transmitted in the parallel waveguides is not absorbed in signal electrode 20 and ground electrode 22.
In FIG. 2A, an optical signal is input into the incident side of waveguide 10 (Opt In). To function as an optical modulator, a microwave signal being output from a signal source 25 is supplied to signal electrode 20 as a modulation signal in the same direction as the above-mentioned optical signal transmission direction. Accordingly, a refraction index of the branching parallel optical waveguide varies in mutually opposite direction corresponding to the polarity of the microwave signal. This produces an optical phase difference in each parallel optical waveguide, to output an optical signal being intensity-modulated from an output side (Opt Out) of optical waveguide 10 shown in FIG. 2A.
Here, in the optical module configuration shown in FIG. 1, a high frequency signal i.e. a microwave modulation signal fed from signal source 25 is supplied between signal electrode 20 and ground electrodes 21, 22 through an RF connector 3 consisting of a center conductor 30 and an outer conductor 31.
At this time, in the conventional configuration shown in FIG. 1, center conductor 30 of RF connector 3 is inserted into a sliding contact member 32 to connect with signal electrode 20 of electro-optic effect element 2 by bonding. Also, in this conventional configuration, outer conductor 31 of RF connector 3 is connected to ground electrodes 21, 22 of electro-optic effect element 2 through a bonding wire 23.
However, a sufficient space for inserting a bonding tool was required for this bonding. For this reason, it was inevitable to protrude center conductor 30 of RF connector 3 from the coaxial condition of RF connector 3. This causes increased characteristic impedance in the protrusion portion of center conductor 30 in RF connector 3, resulting in producing impedance mismatch.
Further, though the characteristic is not influenced so much when the wavelength of a high frequency signal is long as compared to the size of the electrode in electro-optic effect element 2, the high frequency characteristic of electro-optic effect element 2 is substantially influenced when the high frequency signal wavelength becomes shorter. As a result radiation and reflection are produced in the high frequency signal, and a wideband transmission characteristic in electro-optic effect element 2 becomes impeded. Also, the sizes of slide contact member 32 and center conductor 30 of RF connector 3 are as minute as several tens xcexcm. Therefore, efficiency in assembly work is significantly bad.
To cope with these problems, the inventors of the present invention have been studying of adopting a method of using an intermediate or repeating board (hereinafter, called as repeating board). However, in the connection between RF connector 3 and electro-optic effect element 2 through the repeating board, even when designing a characteristic impedance to be, for example, 50 xcexa9 in each portion itself, it is inevitable to have an abrupt impedance variation in the connection of RF connector 3 and the repeating board. As a result, still there has been a problem of an increased transmission loss.
Accordingly, in an optical module on which an element for shifting an optical phase by the electro-optic effect is mounted, it is an object of the present invention to provide a preferable configuration of the optical module to circumvent a loss produced when a high frequency control signal is externally supplied through an RF connector.
According to the present invention to solve the aforementioned problem, as a first embodiment of the optical module having an element for shifting an optical phase by the electro-optic effect, the optical module includes; the element for shifting an optical phase by the electro-optic effect having a signal electrode and a ground electrode formed thereupon; a connector for supplying a control signal of a microwave region to the signal electrode of the element, having a center conductor and an outer conductor; and a repeating board formed on a dielectric wafer having the signal electrode and the ground electrode of the element and a coplanar line for connecting the center conductor of the connector with the outer conductor respectively formed on the dielectric wafer. Here, an air layer is formed on the lower portion of the repeating board on which the center conductor of the connector is disposed.
As a second embodiment of the optical module to solve the aforementioned problem, in the first embodiment, the air layer is formed on the lower portion of the repeating board on which the center conductor of the connector is disposed is formed by a notch produced on the side face of the dielectric wafer positioned oppositely to the connector.
As a third embodiment of the optical module to solve the aforementioned problem, in the second embodiment, the coplanar line is constituted by a signal electrode and ground electrodes disposed on both sides of the signal electrode. The interval between the ground electrodes is smaller than the diameter of the outer conductor of the connector.
As a fourth embodiment of the optical module to solve the aforementioned problem, the optical module includes; the element for shifting an optical phase produced by the electro-optic effect, having a signal electrode and a ground electrode formed thereupon; a connector for supplying a control signal of a microwave region to the signal electrode of the element, having a center conductor and an outer conductor; and a repeating board formed on a dielectric wafer having the signal electrode and the ground electrode of the element and a coplanar line for connecting the center conductor of the connector with the outer conductor respectively formed on the dielectric wafer. The outer conductor of the connector has an extended diameter in the area positioned oppositely to the repeating board.
As a fifth embodiment of the optical module to solve the aforementioned problem, the optical module includes; the element for shifting an optical phase by the electro-optic effect having a signal electrode and a ground electrode formed thereupon; a connector for supplying a control signal of a microwave region to the signal electrode of the element, having a center conductor and an outer conductor; and a repeating board formed on a dielectric wafer having the signal electrode and the ground electrode of the element and a coplanar line for connecting the center conductor of the connector with the outer conductor respectively formed on the dielectric wafer. Here, the lower portion of the repeating board on which the center conductor of the connector is disposed is chamfered in a taper shape to form an air layer.
As a sixth embodiment of the optical module to solve the aforementioned problem, in the fourth or fifth embodiment, the coplanar line is constituted by a signal electrode and ground electrodes disposed between both sides of the signal electrode. The interval between the ground electrodes is larger than the diameter of the outer conductor of the connector.
As a seventh embodiment of the optical module to solve the aforementioned problem, the optical module includes; the element for shifting an optical phase by the electro-optic effect having a signal electrode and a ground electrode formed thereupon; a connector for supplying a control signal of a microwave region to the signal electrode of the element, having a center conductor and an outer conductor; a repeating board formed on a dielectric wafer having the signal electrode and the ground electrode of the element and a coplanar line for connecting the center conductor of the connector with the outer conductor respectively formed on the dielectric wafer; and a metal body for mounting the dielectric wafer for the repeating board. Here, an air layer is formed on the metal body portion corresponding to the lower portion of the repeating board on which the center conductor of the connector is disposed.
As an eighth embodiment of the optical module to solve the aforementioned problem, the optical module includes; the element for shifting an optical phase by the electro-optic effect having a signal electrode and a ground electrode formed thereupon; a connector for supplying a control signal of a microwave region to the signal electrode of the element, having a center conductor and an outer conductor; a repeating board formed on a dielectric wafer having the signal electrode and the ground electrode of the element and a coplanar line for connecting the center conductor of the connector with the outer conductor respectively formed on the dielectric wafer; and a metal body for mounting the repeating board formed on the dielectric wafer. Here, the dielectric wafer for the repeating board is electrically connected to the metal body through a plurality of VIA.
As a ninth embodiment of the optical module to solve the aforementioned problem, in the eighth embodiment, the dielectric wafer is constituted of multi-layer structure having an internal conductor in the middle layer to connect the plurality of VIA by the internal conductor.
As a tenth embodiment of the optical module to solve the aforementioned problem, among the plurality of VIA in the ninth embodiment, a VIA of the repeating board positioned in the area in which the center conductor of the connector is disposed is not connected to the internal conductor.
As an eleventh embodiment of the optical module to solve the aforementioned problem, in the first, fourth or fifth embodiment, the width of the signal electrode of the coplanar line on the repeating board being connected to the center conductor of the connector is smaller than the diameter of the center conductor.
As a twelfth embodiment of the optical module to solve the aforementioned problem, in the first, fourth or fifth embodiment, the signal electrode of the coplanar line on the repeating board being connected to the center conductor of the connector is lozenge-shaped having both a taper portion extending in the direction toward the connector and a tape portion narrowing toward the connector. Also each ground electrode being disposed on both sides of the signal electrode has a taper portion corresponding to the taper portion extending toward the connector.
As a thirteenth embodiment of the optical module to solve the aforementioned problem, in the first, fourth or fifth embodiment, the signal electrode width of the coplanar line is smaller than the diameter of the center conductor of the connector.
Further scopes and features of the present invention will become more apparent by the following description of the embodiments with the accompanied drawings.